Nano-Structures and Interactions of Alkali Metals within Silica Gel

Liquid alkali metals and their alloys can be absorbed into the 15 nm diameter pores of nanostructured silica gel (SG) at loadings up to 40 wt %. Characterization of the resultant materials was done by measuring the amount of hydrogen produced by addition of alcohols and/or water, by differential scanning calorimetry, and by static Na-23 NMR spectroscopy. While sodium must be heated above 100 degrees C to form Na in SG, absorption of Na-K alloys at ambient temperatures yields nanoscale metallic clusters in the pores (Stage 0). In both cases, some of the encapsulated metal may dissolve in the silica and ionize to produce highly mobile M+ ions with reversible electron transfer to the silica. This presumption is consistent with literature observations that sodium is soluble in silica films at ambient temperatures at levels up to 35 mol percent and is presumed to form highly mobile Na+-O- species (Lagarde, P.; Flank, A.-M.; Mazzara, C.; Jupille, J. Surf Sri. 2001, 482-485, 376-380). Heating M-SG materials leads to reduction of SiO2 to form material that contains localized M+-O- species and Si Si bonds in increased proportion as the temperature and time of heating are increased. Above 350 degrees C, elemental silicon, alkali metal silicides, and alkali metal silicates are formed. The dissolution and reaction of potassium from encapsulated Na-K alloys occurs preferentially over that of sodium. Irreversible reaction even occurs at room temperature over a period of months. The temperature dependence of solid state Na-23 NMR spectra indicates the presence of rapid exchange between Na-0 in the metal clusters and Na+ at the walls (and perhaps within the silica).